I've been studying this morning for my Anatomy test. I prefer the little video links that Dr. Endres gives to us, but I think I've listened to this particular part of the video about ten times so far:

"The depolarization of the T-tubule membrane causes nearby voltage-gated calcium ion channels on the sarcoplasmic reticulum to open, resulting in an increase in the permeability of the sarcoplasmic reticulum to calcium ions. Calcium ions then diffuse from the sarcoplasmic reticulum into the sarcoplasm. Long filamentous tropomyosin proteins lie on two sides of the Actin myofilament, covering up the sites where myosin can bind to the Actin myofilament. Globular troponin proteins are attached to the tropomyosin. When calcium ions bind to troponin, this causes troponin to change confirmation and move the tropomyosin. When the tropomyosin moves, this uncovers the myosin binding sites on the Actin, allowing myosin heads to bind and form cross-bridges."

*Well, typing it actually helped*